Device having thyristors connected in series

Abstract

A device comprising a series circuit consisting of a plurality of thyristors connected in series, and a reverse-blocking semiconductor element such as a diode connected in an antiparallel relationship to the series circuit. The device further includes means for detecting whether a sufficient turn-off time is secured for each of the thyristors anterior or posterior to a reference moment at the zero point of a current flowing in the diode, and means for turning on the entirety of the thyristors when the sufficient turn-off time is not secured. Since none of the thyristors is turned off in the state where the turn-off time is insufficient, it becomes possible with certainty to achieve exact prevention of a fault that one or more thyristors, turned off earlier than others are led to breakdown by an overvoltage.

What is claimed is: 
     
       1. A device having series connected thyristors, comprising: a current controller including a series circuit consisting of a plurality of thyristors connected in series, and a reverse-blocking semiconductor element connected to said series circuit in an antiparallel relationship; a gate signal generator for feeding a gate signal to the gate of each of said thyristors; and a thyristor protective unit consisting of a circuit for detecting a zero point of the current flowing in said reverse-blocking semiconductor element and generating a zero detection pulse, a monostable multivibrator for generating a pulse of a predetermined duration synchronously with the time of extinction of said gate signal, an AND element so connected as to receive said two pulses, and means for feeding the output of said AND element as a reignition pulse to said gate signal generator. 
     
     
       2. The device as defined in claim 1, wherein said circuit for generating a zero detection pulse is equipped with a conduction period detector capable of generating an output only during the conduction period of said reverse-blocking semiconductor element, and a monostable multivibrator for generating a zero detection pulse synchronized with the fall of the output of said conduction period detector. 
     
     
       3. A device having series connected thyristors, comprising: a current controller including a series circuit consisting of a plurality of thyristors connected in series, and a reverse-blocking semiconductor element connected to said series circuit in an antiparallel relationship; a gate signal generator for feeding a gate signal to the gate of each of said thyristors; and a thyristor protective unit consisting of a conduction period detector for producing a detection signal merely during the conduction period of said reverse-blocking semiconductor element, comparison means for comparing the duration of the detection signal of said conduction period detector with a predetermined set time and producing an output when the former is shorter than the latter, and means for feeding to said gate signal generator the output of said comparison means as a reignition pulse synchronously with extinction of the detection signal of said conduction period detector. 
     
     
       4. The device as defined in claim 3, wherein said comparison means consists of an integrator for integrating the output signal of said conduction period detector, means resettable at the fall of the detection signal of said conduction period detector, and a comparator for comparing the integral signal of said integrator with a preset reference value. 
     
     
       5. A device having series connected thyristors, comprising: a current controller including a series circuit consisting of a plurality of thyristors connected in series, and a reverse-blocking semiconductor element connected to said series circuit in an antiparallel relationship; a gate signal generator for feeding a gate signal to the gate of each of said thyristors; and first and second protective units for said thyristors, wherein said first protective unit consists of a circuit for detecting a zero point of the current flowing in said reverse-blocking semiconductor element and generating a zero detection pulse, a monostable multivibrator for generating a pulse of a predetermined duration synchronously with the time of extinction of said gate signal, an AND element so connected as to receive said two pulses, and means for feeding the output of said AND element as a reignition pulse to said gate signal generator; and said second protective unit consists of a conduction period detector for producing a detection signal merely during the conduction period of said reverse-blocking semiconductor element, a comparison means for comparing the duration of the output of said conduction period detector with a predetermined set time and producing an output when the former is shorter than the latter, and means for feeding to said gate signal generator the output of said comparison means as a reignition pulse synchronously with extinction of the output signal of said conduction period detector.